Does inactivating the PCSK9 enzyme lower LDL levels in humans? Researchers inter

Question

Does inactivating the PCSK9 enzyme lower LDL levels in humans?

Researchers interested in genetic factors affecting susceptibility to cardiovascular disease examined the DNA of 15,000 individuals. This screening revealed that 3% of the population sample had a mutation that inactivated one copy (allele) of the gene for PCSK9, a human liver enzyme. Because mutations that increase PCSK9 activity increase levels of LDL cholesterol in the blood, the researchers hypothesized that inactivating mutations in this gene would lower LDL levels. In this exercise, you will examine the results of an experiment they carried out to test this hypothesis.

The researchers measured LDL cholesterol levels in blood plasma from 85 individuals with one copy of the PCSK9 gene inactivated (the study group) and from 3,278 wild-type individuals, who had two functional copies of the PCSK9 gene (the control group).

What is the best explanation for the fact that the two histograms overlap as much as they do?

-PCSK9 enzyme activity varies widely among individuals with an inactivating mutation in only one copy of the PCSK9 gene. -The sample size of the study group is much smaller than the sample size of the control group. -Many factors in addition to PCSK9 enzyme activity affect plasma LDL levels. -Plasma LDL levels are hard to measure precisely, so the histograms show the spread around the true value. 30 20 10 0 50 100 150 200 250 300 Plasma LDL cholesterol (mg/dL) Individuals with an inactivating mutation in one copy of PCSK9 gene (study group) 30 20 10 0 50 100 150 200 250300 Plasma LDL cholesterol (mg/dL) Individuals with two functional copies of PCSK9 gene (control group) Data from J. C. Cohen et al., Sequence variations in PCSK9, low LDL, and protection against coronary heart disease, New England Journal of Medicine 354:1264-1272 (2006)

Explanation / Answer

Many factors in addition to PCSK9 enzyme activity affect plasma LDL levels

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